Carbon dioxide (CO2) accounts for about 76% of global greenhouse gas emissions from human activities, with methane (16%), nitrous oxide (6%), and fluorinated gases (2%) accounting for the balance (United States Environmental Protection Agency). The majority of CO2 comes from the burning of fossil fuels to produce energy, although industrial and forestry practices also emit CO2 into the atmosphere. Reduction of greenhouse gas emissions, particularly CO2, is critical to halt the progression of global warming and the accompanying shifts in climate and weather.
It has long been recognized that catalytic processes, such as the Fischer-Tropsch process, may be used to convert gases containing carbon dioxide (CO2), carbon monoxide (CO), and/or hydrogen (H2), such as industrial waste gas or syngas, into a variety of fuels and chemicals. Recently, however, gas fermentation has emerged as an alternative platform for the biological fixation of such gases. In particular, C1-fixing microorganisms have been demonstrated to convert gases containing CO2, CO, and/or H2 into products such as ethanol and 2,3-butanediol. The production of such products may be limited, for example, by slow microbial growth, limited gas consumption, sensitivity to toxins, or diversion of carbon substrates into undesired by-products.
The accumulation of products can result in a reduction in the production efficiency of the gas fermentation process. To prevent accumulation, these products must be removed at an effective rate. If not removed at an effective rate, these products may have inhibitory and/or toxic effects on the C1-fixing microorganisms. If the products accumulate to the point that the C1-fixing microorganisms cannot survive, then the fermentation process may have to be stopped and restarted. Prior to being restarted, the fermenters often require cleaning. This can be a time-consuming process.
Another pitfall commonly associated with the recovery of products is the loss of C1-fixing microorganisms through traditional recovery processes. To overcome the loss of viable C1-fixing microorganisms, filtration methods have been employed. However, over time, with traditional filtration methods, particulate matter can build up in the filter media, which can lead to a reduction in the filtrate flux, ultimately requiring cleaning and/or replacement of the filter media.
Accordingly, there remains a need for a system with reduced maintenance requirements that is capable of recovering products at an effective rate while ensuring the viability of the C1-fixing microorganisms.